A MRAM is a promising nonvolatile memory from a viewpoint of high integration and a high-speed operation. In the MRAM, a “magnetoresistance effect element” that exhibits a magnetoresistance effect such as a TMR (Tunnel MagnetoResistance) effect is used as a memory cell. In the magnetoresistance effect element, for example, a MTJ (Magnetic Tunnel Junction) in which a tunnel barrier layer is sandwiched by two ferromagnetic layers is formed. The two ferromagnetic layers include a magnetization fixed layer (pinned layer) whose magnetization direction is fixed and a magnetization free layer (free layer) whose magnetization direction is reversible.
It is known that a resistance value (R+ΔR) of the MTJ when the magnetization directions of the magnetization fixed layer and the magnetization free layer are “anti-parallel” to each other becomes larger than a resistance value (R) when they are “parallel” to each other, because of the magnetoresistance effect. An MR ratio (=ΔR/R) at room temperature is a few tens to a few hundreds of %. A memory cell of the MRAM utilizes the change in the resistance value to nonvolatilely store data. Data reading is performed by flowing a read current so as to penetrate through the MTJ and measuring the resistance value of the MTJ. On the other hand, data writing is performed by switching the magnetization direction of the magnetization free layer.
As a typical data writing method, a “current magnetic field method” is known. According to the current magnetic field method, a write current is flowed through a write interconnection arranged in the vicinity of the magnetoresistance effect element. Then, a write magnetic field generated by the write current is applied to the magnetization free layer and thereby the magnetization direction of the magnetization free layer is changed. At this time, a strength of the magnetic field generated by the write current of 1 mA is approximately a few Oe to dozen Oe. Meanwhile, to prevent stored data from being rewritten by thermal disturbance, a magnetic switching field necessary for switching the magnetization of the magnetization free layer is preferably designed to be about a few tens of Oe. Therefore, it is very difficult to achieve the data writing with a write current of 1 mA or less. In this point, the MRAM based on the current magnetic field method is less advantageous than the other RAM. Furthermore, the magnetic switching field necessary for switching the magnetization of the magnetization free layer increases in substantially inverse proportion to a size of the magnetoresistance effect element. That is to say, the write current tends to increase with the miniaturization of the memory cell, which is a problem.
With regard to the MRAM based on the current magnetic field method, Japanese Laid-Open Patent Application No. JP-2005-150303 describes a technique intended to improve thermal disturbance resistance and to reduce the magnetic switching field. A magnetoresistance effect element according to the technique has a ferromagnetic tunnel junction including a three-layer structure of a first ferromagnetic layer/a tunnel barrier layer/a second ferromagnetic layer. The first ferromagnetic layer has larger coercive force as compared with the second ferromagnetic layer. Further, magnetization of an end of the second ferromagnetic layer is fixed in a direction having a component orthogonal to a direction of easy magnetization axis of the second ferromagnetic layer.
As a data write method replacing the current magnetic field method, a “spin transfer method” using spin transfer is recently proposed. Refer to, for example, Japanese Laid-Open Patent Application No. JP-2005-191032 and a literature (M. Hosomi, et al., “A Novel Nonvolatile Memory with Spin Torque Transfer Magnetization Switching: Spin-RAM”, International Electron Devices Meeting (IEDM) Technical Digest, pp. 459-562, 2005). According to the spin transfer method, a spin-polarized current is injected into a magnetization free layer, and direct interaction between spin of conduction electrons of the current and magnetic moment of the conductor causes the magnetization to be switched. Since the magnetization switching is more likely to occur as a current density increases, it is possible to decrease the write current as the memory cell size is reduced.
According to the spin transfer method described in the literature (M. Hosomi, et al., “A Novel Nonvolatile Memory with Spin Torque Transfer Magnetization Switching: Spin-RAM”, International Electron Devices Meeting (IEDM), Technical Digest, pp. 459-562, 2005), the write current is so flowed as to penetrate through the MTJ. That is, the spin transfer method according to the technique is achieved by a so-called CPP (Current Perpendicular to Plane) method, which is hereinafter referred to as a “vertical spin transfer method”. According to the vertical spin transfer method, spin-polarized electrons having the same spin state as that of the magnetization fixed layer are supplied from the magnetization fixed layer to the magnetization free layer, or extracted from the magnetization free layer to the magnetization fixed layer. As a result, the magnetization of the magnetization free layer is reversed due to the spin transfer effect. In this manner, the magnetization direction of the magnetization free layer can be determined depending on a direction of the write current penetrating through the MTJ. Furthermore, it is possible to reduce the write current with the miniaturization of the memory cell.
However, according to the vertical spin transfer method, the write current larger than the read current penetrates through the MTJ, which is considered to cause the following problems. Typically, an insulating film is used as the tunnel barrier layer of the MTJ, and an upper limit value of the write current is determined by a limit of a breakdown voltage of the insulating film. This is not preferable from a view point of writing. On the other hand, if a resistance value of the tunnel barrier layer is lowered in order to increase the upper limit value, it causes decrease in a read signal. This is not preferable from a view point of reading. That is to say, the writing should be performed within a margin more than a current causing the magnetization switching and less than the breakdown voltage of the insulating film meeting read constraint, which is disadvantageous. Furthermore, to flow the write current through the insulating film every time the write operation is performed is not preferable from a view point of durability of the element.
On the other hand, according to the spin transfer method described in Japanese Laid-Open Patent Application No. JP-2005-191032, the write current does not penetrate through the MTJ but flows within the plane of the magnetization free layer. This spin transfer method is hereinafter referred to as a “horizontal spin transfer method”. More specifically, the magnetization free layer according to this technique has a connector section overlapping with the tunnel barrier layer, constricted sections adjacent to both ends of the connector section, and a pair of magnetization fixed sections respectively formed adjacent to the constricted sections. The magnetization fixed sections are respectively provided with fixed magnetizations whose directions are opposite to each other. As a result, the magnetization free layer is provided with a domain wall within the above-mentioned connector section.
Within the magnetization free layer thus constructed, the write current is flowed planarly. At this time, the pair of magnetization fixed sections serves as supply sources of different types of spin polarized electrons. A direction of the write current is controlled depending on a write data, and the spin polarized electrons are supplied to the connector section from any of the magnetization fixed sectors depending on the direction. As a result, the magnetization of the magnetization free layer is reversed due to the spin transfer effect. The magnetization reversal means a change in position of the above-mentioned domain wall. That is, the domain wall moves between the pair of constricted sections, depending on the direction of the write current. In this sense, the horizontal spin transfer method as described in Japanese Laid-Open Patent Application No. JP-2005-191032 can also be referred to as a “domain wall motion method”.
Such a current-driven domain wall motion has been actually observed in a ferromagnetic fine wire (see Yamaguchi et. al., “Real-Space Observation of Current-Driven Domain Wall Motion in Submicron Magnetic Wires”, PRL, vol. 92, pp. 077205-1, 2004). When a current crossing a domain wall flows through the magnetic fine wire that has the domain wall and a width of a few tens nanometers to a few micrometers, the domain wall is moved by spin magnetic moment of the conduction electrons. A current value required for the domain motion also decreases with the miniaturization of the element. Accordingly, the horizontal spin transfer method (domain wall motion method) utilizing the domain wall motion is a very important technique for achieving a large-capacity low-current-operation MRAM.